Charge carrier localization induced by excess Fe in the Fe$_{1+y}$(Te$_{1-x}$Se$_{x})$ superconductor system

Iron chalcogenide Fe$_{1+y}$(Te$_{1-x}$Se$_{x})$ is the simplified version of Fe-based superconductors. Its non-superconducting parent compound Fe$_{1+y}$Te exhibits an AFM structure distinct from those seen in undoped FeAs compounds. Understanding of the superconducting properties of this system has been considered critical. We have investigated the effect of Fe nonstoichiometry on properties of the Fe$_{1+y}$(Te$_{1-x}$Se$_{x})$ superconductor system by means of resistivity, Hall coefficient, magnetic susceptibility, and specific heat measurements. We find that the excess Fe at interstitial sites of the (Te, Se) layers not only suppresses superconductivity, but also results in a weakly localized electronic state. Together with neutron scattering studies and recent DFT calculations, our results suggest that such weak charge carrier localization originates from the magnetic coupling between the excess Fe and the adjacent Fe square planar sheets.